Major Depressive Disorder (MDD) is projected to be one of the leading causes of disability in the world. The phenotypic variability of MDD impairs the diagnosis of this disease in large populations and inhibits the development of novel therapies. In addition, symptoms of MDD are not equally ameliorated by treatments such as selective serotonin reuptake inhibitors (SSRIs). Among the symptoms of MDD, anhedonia—defined as diminishment of interest or pleasure in stimuli that were once rewarding—is one of the most common symptoms, occurring in nearly 40% of all MDD cases. It is also one of the most difficult symptoms to treat. Because anhedonia symptoms may predict poor treatment response to overall MDD, methods are needed to specifically detect anhedonia, especially in the context of MDD.
Several barriers exist to the detection, diagnosis, and monitoring of MDD. Most current diagnostic tools for measuring MDD rely on a summation of a constellation of symptoms across days or weeks. These tools are insufficient for detection of specific symptoms, such as anhedonia, as there are typically a small number of questions dedicated to a specific symptom. For example, the Hamilton Depression Rating Scale and Beck Depression Inventory, two commonly used depression instruments, have only one question and four questions, respectively, targeting anhedonia. Furthermore, psychometric studies suggest that anhedonic symptoms and depressive symptoms are only moderately associated.
Common screening instruments do not distinguish between the neuropsychological constructs of anhedonia, consummatory and anticipatory anhedonia. Specifically, consummatory anhedonia describes the loss of momentary pleasure, for example while engaged in an activity that would ordinarily be pleasurable. Anticipatory anhedonia, on the other hand, describes the loss of future pleasure, for example the anticipated amount of pleasure expected from a future activity. Separate measures for these constructs may provide clinically valuable information, as depressed individuals often will have similar consummatory pleasures as non-depressed individuals, but will have impaired anticipatory pleasure. This impairment may affect decision-making, and measuring the subtypes of anhedonia may guide effective treatments.